The appended drawing, FIG. 1A illustrates a conventional way of biasing the gate G of an RF power LDMOS field effect transistor 1 to a gate voltage VG that gives a desired value of quiescent current IDQ of the transistor 1. The RF signal is supplied to the gate G via a terminal 2.
To bias the gate G of the transistor 1, a fixed resistor R1 is connected between the gate G and the source S, which normally is connected to ground, and a variable resistor R2 is connected between the gate G and a terminal at a positive voltage. By means of the resistor R2, the gate voltage VG is adjusted to a value that gives the desired quiescent current IDQ through the transistor 1.
The value of the quiescent current IDQ is commonly chosen to give a flat gain versus output power characteristic. Any deviation from this chosen IDQ value will degrade the linearity performance of the transistor.
For a given value of the gate voltage VG, the quiescent current IDQ is temperature dependent. Consequently, temperature changes will cause degradation of the performance of the transistor 1.
The temperature coefficient of the transistor's quiescent current is a function of the gate bias voltage. The relatively low values of the gate bias voltage VG that are normally used, gives a positive temperature coefficient, i.e. the quiescent current IDQ increases with temperature.
A common approach to reduce the variation of the quiescent current IDQ with changes in temperature is to introduce a discrete diode DI in series with the resistor R1 as shown in FIG. 1B. The voltage drop across the diode decreases as the temperature increases and thus partially eliminates the RF transistor's quiescent current temperature dependence.
There are however two apparent drawbacks of using a diode for temperature compensation. Firstly, the temperature characteristic of a diode does not exactly track the temperature characteristic of an RF LDMOS transistor. Secondly, it is hard to achieve a good thermal coupling between a discrete diode and the transistor, resulting in different temperatures for the two components.